U.S. Pat. No. 3,283,092 discloses a liquid level indicator that includes a housing for mounting on a liquid containment vessel and a hollow conduit that extends from the housing into the vessel. The lower end of the conduit is positioned within the vessel so as to be engaged by liquid as the liquid rises and falls within the vessel. When the liquid within the vessel covers the open lower end of the conduit, air is captured within the conduit, and air pressure within the conduit increases as liquid level increases. A pressure sensor within the housing is coupled to the upper end of the conduit for indicating level of material when the pressure of air captured within the conduit increases to the set point of the pressure sensor.
The apparatus disclosed in the noted patent is thus a point-level sensor for indicating that the level of material within the vessel has reached a point that corresponds to the pressure sensing threshold of the pressure sensor. However, the principles disclosed in the noted patent have also been employed in continuous-type liquid level sensors in which the conduit extends vertically through the vessel between an open lower end positioned adjacent to the bottom of the vessel, and an upper end coupled to a suitable mechanism for indicating level of liquid within the vessel as a continuous function of pressure of air captured within the conduit. Devices of this character are disclosed, for example, in U.S. Pat. Nos. 2,121,743 and 3,221,51. A problem arises in systems of this character, however, due to temperature fluctuations of the air captured within the conduit. That is, temperature drift and non-linearities caused by temperature-induced variations in air pressure within the conduit can cause errors on the order of 0.3%/.degree.C. in conventional continuous-type systems of this character. It is a general object of the present invention to provide a continuous-type liquid level measurement system of the described character that is so constructed as to substantially reduce the effects of temperature variations in the captured air column.